Cryosurgery is a well established modality useful in the treatment of many conditions. Cooling and defrosting may be achieved by a variety of methods. Considering only probe-type instruments (as compared with external, usually dermatological, applications such as dry ice or liquid nitrogen applied directly to the skin), these may include: the introduction of a low boiling point refrigerant into the probe tip; gas expansion utilizing the Joule-Thompson effect; employing the latent heat of vaporization, as with Freon; precooled gases and liquids; or thermoelectric cooling.
It has also been proposed to provide flexible cryoprobes. One such probe is illustrated in U.S Pat. No. 3,859,986 of Okada, et al. Another is illustrated in U.S. Pat. No. 3,910,277 of Zimmer. However, neither of those devices would be suitable for introduction into, and subsequent passage through, a narrow curved passage such as a vein or artery. This is because they could not be made thin enough or flexible enough for such an application. For example, Okada, et al. employs a liquid refrigerant such as liquid nitrogen. The refrigerant is supplied through a flexible supply tube contained within a flexible outer tube. The inner tube must be thermally insulated from the outer tube. Accordingly, spacing ribs are required to separate them and permit the intervening space to be evacuated. Zimmer also employs liquid nitrogen in a tube-within-a-tube construction, but requires interlocking insulation rings, resulting in a relatively thick insulated tube.
U.S. Pat. No. 3,512,531 of Crump et al. is designed for insertion into an artery. However, it is employed for removing the diseased inner lining (or intima) which has been detached from the arterial wall. Accordingly, it is designed to be cooled along one side of its entire length, rather than at the end, in order to permit cryoadhesion along the length of the detached lining. Furthermore, although it is slightly flexible, it is not designed to be threaded through a blood vessel or similar passage to perform cryosurgery, for example within the heart.
Also relevant to the disclosure of this invention is the fact that the method of cooling and defrosting the cryoprobe is known. It comprises supplying a gas such as nitrous oxide (N.sub.2 O) or carbon dioxide (CO.sub.2) from a room temperature bottle through a delivery system. The delivery system supplies the gas to the cryoprobe at either of a high pressure or a low pressure. When supplied at a high pressure, the gas expands through a restricted orifice in the freeze tip and cools by the Joule-Thompson effect. When supplied at a low pressure, the warm gas merely floods the tip and thaws it.